The pharmacokinetics of subcutaneously administered insulin is dependent on its association behavior. Insulin forms hexamers in neutral aqueous solution. To get from the tissue into the blood stream and to the site of action, insulin must first pass through the walls of the capillaries. It is assumed that this is only possible for monomeric and for dimeric insulin--but not possible or only slightly possible for hexameric insulins or relatively high molecular weight associates (Brange et al., Diabetes Care: 13 (1990), pages 923-954; Kang et al., Diabetes Care: 14 (1991), pages 942-948). The dissociation of the hexamer is therefore a prerequisite for rapid passage from the subcutaneous tissue into the blood stream.
The association and aggregation behavior of insulin is affected by zinc.sup.++, which leads to a stabilization of the hexamer and at pHs around the neutral point to the formation of relatively high molecular weight aggregates until precipitation occurs. Zinc.sup.++ as an additive to an unbuffered human insulin solution (pH 4), however, only slightly affects the profile of action. Although such solution is rapidly neutralized in the subcutaneous tissue on injection and insulin-zinc complexes are formed, the natural zinc binding of human insulin is insufficient to stabilize hexamers and higher aggregates. Therefore, by addition of zinc.sup.++, the release of human insulin is not markedly delayed and a strong depot effect is not achieved. Known insulin hexamers have a content of approximately 2 mol of zinc per mole of insulin hexamer (Blundell et al., Adv. Protein Chem.: 26 (1972), pages 323-328). Two zinc molecules per insulin hexamer are firmly bound to the insulin hexamer and cannot be removed by normal dialysis. So-called 4-zinc insulin crystals have been described, but these crystals on average contain only less than three mol of zinc.sup.++ per mole of insulin hexamer (G.D. Smith et al., Proc. Natl. Acad. Sci. USA: 81, pages 7093-7097).